This invention relates to plasma-catalyst systems and, more particularly, to a control system for reducing electrical power consumption in a plasma-catalyst system.
Certain compounds in the exhaust stream of a combustion process, such as the exhaust stream from an internal combustion engine, are undesirable and must be controlled in order to and meet government emissions regulations. Among the regulated compounds are hydrocarbons, soot particulates, and nitrogen oxide compounds (NOx). There is a wide variety of combustion processes producing these emissions; for instance, coal or oil-fired furnaces, reciprocating internal combustion engines (including gasoline spark ignition and diesel engines), gas turbine engines, and so on. In each of these combustion processes, control measures to prevent or diminish atmospheric emissions of these emissions are needed.
Industry has devoted considerable effort to reducing regulated emissions from the exhaust streams of combustion processes. In particular, it is now common in the automotive industry to place a catalytic converter in the exhaust system of gasoline spark ignition engines to remove undesirable emissions from the exhaust by chemical treatment. Typically, a xe2x80x9cthree-wayxe2x80x9d catalyst system of platinum, palladium, and/or rhodium metals dispersed on an oxide support is used to oxidize carbon monoxide and hydrocarbons to water and carbon dioxide and to reduce nitrogen oxides to nitrogen. The catalyst system is applied to a ceramic substrate such as beads, pellets, or a monolith. When used, beads are usually porous, ceramic spheres having the catalyst metals impregnated in an outer shell. The beads or pellets are of a suitable size and number in the catalytic converter in order to place an aggregate surface area in contact with the exhaust stream that is sufficient to treat the compounds of interest. When a monolith is used, it is usually a cordierite honeycomb monolith and may be pre-coated with .gamma.-alumina and other specialty oxide materials to provide a durable, high surface area support phase for catalyst deposition. The honeycomb shape, used with the parallel channels running in the direction of the flow of the exhaust stream, both increases the surface area exposed to the exhaust stream and allows the exhaust stream to pass through the catalytic converter without creating undue back pressure that would interfere with operation of the engine.
When a spark ignition engine is operating under stoichiometric conditions or nearly stoichiometric conditions with respect to the fuel/air ratio (just enough oxygen to completely combust the fuel, or perhaps up to 0.3% excess oxygen), a xe2x80x9cthree-wayxe2x80x9d catalyst has proven satisfactory for reducing emissions. Unburned fuel (hydrocarbons) and oxygen are consumed in the catalytic converter, and the relatively small amount of excess oxygen does not interfere with the intended operation of the conventional catalyst system. The stoichiometric conditions or nearly stoichiometric conditions will be referred to as non-oxidizing conditions or as producing a non-oxidizing atmosphere.
However, it is desirable to operate the engine at times under lean burn conditions, with excess air, in order to improve fuel economy. While conventional non-oxidizing engine conditions might have a fuel-to-air mixture having .1-.3% excess oxygen, with perhaps a slightly greater amount in the exhaust as a result of incomplete combustion, a lean burn engine has a substantially greater excess of oxygen, from about 1% to perhaps up to 10% excess oxygen relative to the amount of fuel. Under lean burn conditions, conventional catalytic devices are not very effective for treating the NOx in the resulting oxygen-rich exhaust stream. Lean burn conditions will be referred to as oxidizing conditions or as producing an oxidizing atmosphere. The exhaust stream from a diesel engine also has a substantial oxygen content, from perhaps about 2-18% oxygen, and, in addition, contains a significant amount of particulate emissions. The particulate emissions, or soot, are thought to be primarily carbonaceous particles. It is also believed that other combustion processes result in emissions that are difficult or expensive to control because of, for instance, dilute concentrations of the compounds to be removed from the effluent stream or poor conversion of the compounds using conventional means.
In spite of efforts over the last decade to develop effective means for reducing NOx to nitrogen under oxidizing conditions in a spark ignition gasoline engine or in a diesel engine, the need for improved conversion effectiveness has remained unsatisfied. Moreover, there is a continuing need for improved effectiveness in treating emissions from any combustion process, particularly for treating the soot particulate emissions from diesel engines.
An alternative way to treat the hydrocarbon, particulate, or NOx emissions in an exhaust or effluent stream would be to destroy such emissions using a non-thermal plasma combined with a suitable catalyst (plasma-catalyst). It is known that plasma-catalyst reactors are useful to treat NOx and SOx emissions in power plant flue gases, and even to treat NOx or particulate emissions in diesel engine exhaust. However, systems now known in the art suffer from serious shortcomings. For example, such systems are run continually, which results in relatively large power consumption per unit of material destroyed, particularly when used to treat low concentration of emissions in effluent or exhaust streams.
FIGS. 1 and 2 show NOx mass flow rate (g/s) for each second during the FTP and MVEURO cycles respectively, plotted against measured exhaust volume flow (L/s). It can be seen that there is little relation between the two. FIGS. 3 and 4 show NOx concentration (ppm) versus exhaust flow. Here, it can be seen that there are modes with high exhaust flow but low NOx concentration. The present invention is directed to taking advantage of these relationships to reduce electrical power consumption by reducing plasma power during such modes.
In accordance with the present invention, a system and method of controlling power to a plasma-catalyst is provided that controls the plasma power in accordance with either measured or estimated values of engine or vehicle operational parameters in order to optimize emission reduction versus energy cost. A basic strategy delivers a constant energy to the plasma per standard volume (or mass) of engine exhaust, usually expressed as constant J/L. This improves energy efficiency over a constant plasma power delivery since the chemical conversion of a plasma-catalyst system is well known to vary with energy per volume, usually measured in Joules energy per Liter of exhaust. An added strategy varies the J/L energy delivery in relation to a measured or estimated value of a relevant parameter. For instance, engine-out and/or tailpipe NOx concentration is measured or estimated, and under conditions of high NOx production, higher energy deposition is commanded; while under lower NOx conditions, lower energy is commanded. This results in an improved energy cost versus emission performance.